electron beam therapy - jpneylon.github.io

Electron Beam Therapy

Dimitre Hristov

http://med.stanford.edu/irt/web/image_catalogue/som_logos/som_logo-dk300.jpg

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Electron Beam Therapy 2009

ASTRO’s view on physics teaching

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Select the correct answer:

The percent depth dose at 5 cm for a 6 MeV beam is approximately: 100%, 90%, 80%, 50%, <5%?

To treat a lesion extending to a depth of 5 cm you will use: 6 MeV, 10 MeV, 15 MeV, 20 MeV?

What is the approximate range of a 6 MeV beam in lung overlaying 1 cm of tissue: 1-3 cm, 7-9 cm, > 12 cm?

A 4x8 cut-out is placed in a 10x10 beam cone for 20 MeV treatment. What changes is: 90% depth, cGy/MU, d_max, skin dose, all of the preceding?

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Outline

• Production of electron beams

• Interactions of electron beams with matter

• Electron beam characteristics

• Treatment planning

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Electron Beam Production

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Field Flatness and Uniformity

Primary scattering foil

Secondary scattering foil

Primary collimator

Secondary collimator

Dual foil system for producing a uniform electron beam

Monoenergetic e– beam

Energy spectrum broadened

Energy spectrum broadened

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Primary Scattering Foil

Secondary Scattering Foil

Ionization Chamber

Photon Jaws

Electron Cone

Electron Beam Production

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Charged particle interactions

Incoming radiation

Bremsstrahlung

Main e- track

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Electron-Matter Interactions

Collisional losses Inelastic (excitation, ionization)

Elastic

col

S

ρ

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Electron-Matter Interactions

Radiative losses :Bremsstrahlung

Total mass stopping power radcoltot

SSS

+

=

ρρρ

rad

S

ρ

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Electron Scattering

Pencil electron beam tracks in a bubble chamber

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Relation of Absorbed Dose to Fluence

med

Φ is the differential fluence in energy of identical

charged particles L/ρ is the restricted mass collision stopping power with a

cutoff energy

( ) dEELDcol

E

E∆∆

Φ= ∫

,

max

ρ

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Electron Dose Measurement

colkcolkk dx

dTSD,,

1

=

Φ=

ρρ colmedcolmedmed dx

dTSD,,

1

=

Φ=

ρρ

colkcolmedk

med SSD

D

,,

=

ρρ

med med

Bragg-Gray Cavity Theory

colkcolkk dx

dTSD,,

1

=

Φ=

ρρ colmedcolmedmed dx

dT1SD,,

=

Φ=

ρρ

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The Bragg-Gray Cavity Theory

The ionization produced in a gas-filled cavity placed in a medium is related to the energy absorbed in the surrounding medium.

When the cavity is sufficiently small, electron fluence does not change.

colkcolmedk

med SSD

D

,,

=

ρρ

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The Bragg-Gray Cavity: stopping power ratios

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Energy specification and measurement

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Measure PDD

for a broad beam

=> 20 cm x 20 cm

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( )2

21

21

2p3p210p

MeVcm00250CMeVcm981CMeV220C

RCRCCE−− ===

×+×+=

. ;. ;.

,

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( ) 500 RcmMeV332E ×= /.

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( ) ( ) )/( p0pzp Rz1EE −=

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Electron and Photon Beams

Photon beams indirectly ionizing exponential attenuation

Electron beams directly ionizing definite range

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Electron Depth Dose: Rules of thumb

90%

(E/3.2) cm

80% (E/2.8) cm

50% (E/2.33) cm

R_p (E/1.98) cm

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Electron and Photon Depth Dose

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Depth doses for 20 MeV electrons compared to 15 MeV X-Rays


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6 MeV electrons 6 MeV X-rays


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Central Axis PDDs: field size dependence 6 MeV electrons

2x2

25x25 4x4

Little changes beyond field radius exceeding

0pp E880R ,.≅

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Central Axis PDDs: field size dependence 16 MeV electrons

2 3

4

6 10-25 Little changes beyond field radius exceeding

0pp E880R ,.≅

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Central Axis PDDs: energy dependence

Energy

(MeV) 4 6 9 12 16 20

Surface Dose

(% of maximum) 73.8 74.3 80.0 84.9 90.5 90.8

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Electron Dose Distributions

4x4 field

2x2 field 4x2 field

6 MeV electrons

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Electron Dose Distributions

16 MeV electrons

4x4 field

10x10 field

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Central Axis PDD: SSD dependence

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Dose profiles, at 15 mm depth at Ep,0 = 9 MeV

Profiles: SSD dependence

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Dose Distributions: SSD dependence

Increased penumbra

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Central axis depth dose curves plotted as a function of the angle of incidence of the beam

Angular dependence

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Curved Surface Dose Distributions

Must correct predicted dose distributions for:

• Scatter of electron beam in tissue (angular dependence) • Air gap (inverse square falloff in exposure)

* SSD

Air gap

Water-equivalent materials can be used as “bolus” to smooth out uneven target surfaces

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Tissue Inhomogeneity

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Irregular Surface Dose Distributions

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)( CET1zdDeff −−=

CET: Coefficient of Equivalent Thickness

P

d z

Compact bone CET: 1.65

Lung CET: 0.2-0.25

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Tissue inhomogeneity

uncorrected calculation corrected calculation

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16 MeV electrons


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Lead shielding for electrons

Thickness (mm) = E (MeV)/2 +1

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Electron Backscatter

Solution: place a low atomic number absorber (bolus) on the surface of the shield to absorb the dose due to backscatter

Beam matching

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Summary

Electron beam depth dose characteristics offer advantages for treating superficial lesions

Tissue inhomogeneities, and surface irregularities alter dose distributions significantly.

Modern treatment planning systems and dose-calculation algorithms open the possibility of wider use of electron beams as a treatment modality.

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Select the correct answer:

The percent depth dose at 5 cm for a 6 MeV beam is approximately: 100%, 90%, 80%, 50%, <5%?

To treat a lesion extending to a depth of 5 cm you will use: 6 MeV, 10 MeV, 15 MeV, 20 MeV?

What is the approximate range of a 6 MeV beam in lung overlaying 1 cm of tissue: 1-3 cm, 7-9 cm, > 12 cm?

A 4x8 cut-out is placed in a 10x10 beam cone for 20 MeV treatment. What changes is: 90% depth, cGy/MU, d_max, skin dose, all of the preceding?

electron beam therapy - jpneylon.github.io

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